1. Field of the Invention.
The invention in general relates to fluid detectors and in particular to fluid detectors that are used in combination with polar-non-polar liquid detectors to produce outputs indicative of the presence of air (or dry condition) water and hydrocarbon.
2. Description of the Prior Art.
Thermistors and other devices that have a temperature-dependent electrical property have long been used to differentiate between materials. See for example U.S. Pat. Nos. 3,576,472 issued to John F. Marshall, III, 3,712,116 issued to Marshall L. Andre, 4,116,045 issued to Bronson M. Potter, and 4,221,125 issued to John N. Oliver and Louis M. Sandler. Generally, two thermistors are used, one immersed in the material to be monitored and the other acting as a reference. (See the Marshall, Andre and Potter patents listed above.) Although the prior art two-thermistor-based detection systems work well in the laboratory or manufacturing situations where the environments of the thermistors can be closely controlled, in natural settings, such as the ground around gasoline storage tanks, problems arise in obtaining reliable differentiation between the two thermistors. Thus complicated probe systems and electronics have been developed to attempt to deal with this problem. See the Potter and Oliver et al patents referenced above. Even these complicated solutions have not been wholly reliable, and the hydrocarbon detection industry has generally replaced the thermistor-based systems with systems based on a combination of a float and conductivity sensor. See for example, U.S. Pat. No. 4,586,033. Although such systems generally work well, they have the disadvantages that the probes are relatively large and thus do not fit into small spaces, and the probes must be oriented essentially vertically for the floats to work properly.